sfc: support FEC configuration through ethtool

As well as 'auto' and the forced 'off', 'rs' and 'baser' states, we also
 handle combinations of settings (since the fecparam->fec field is a
 bitmask), where auto|rs and auto|baser specify a preferred FEC mode but
 will fall back to the other if the cable or link partner doesn't support
 it.  rs|baser (with or without auto bit) means prefer FEC even where
 auto wouldn't use it, but let FW choose which encoding to use.

Signed-off-by: Edward Cree <ecree@solarflare.com>
Signed-off-by: David S. Miller <davem@davemloft.net>